The disclosure of Japanese Patent Application Nos. 2000-068553 filed on Mar. 13, 2000 and 2000-169897 filed on Jun. 7, 2000 including the specification, drawings and abstract are incorporated herein by reference in their entirety.
1. Field of the Invention
The invention relates to a fuel cell gas separator, a manufacturing method thereof, and a fuel cell. More particularly, the invention relates to a fuel cell gas separator provided between adjacent single cells in a fuel cell having a plurality of single cells stacked on each other, for forming a fuel gas flow path or an oxidized gas flow path together with an adjacent member and for separating the fuel gas and the oxidized gas from each other, a manufacturing method thereof, and the fuel cell.
2. Description of Related Art
A fuel cell gas separator is a member that forms a fuel cell stack having a plurality of single cells stacked on each other. The fuel cell gas separator has sufficient gas non-permeability in order to prevent the fuel gas and oxidized gas supplied to each of adjacent single cells from mixing together. Conventionally, such a fuel cell gas separator has been manufactured by using a carbon material or metal material. In general, a metal material exhibits higher strength, and therefore makes it possible to manufacture a thinner gas separator as compared to the case using the carbon material. Such a reduced thickness of the gas separator enables reduction in overall size of the fuel cell. Moreover, a metal gas separator can be manufactured by a simple method of pressing a metal sheet. As a result, the manufacturing process can be conducted in a quick, simplified manner, resulting in improved productivity. Thus, increase in manufacturing cost can be prevented.
A metal used for manufacturing the metal gas separator can be selected as appropriate from the metals having sufficient conductivity, strength and formability. In particular, by using a metal that is mass distributed as a metal material like stainless steel and aluminum, significant reduction in manufacturing cost can be achieved. The use of such a metal material normally requires the structure for ensuring sufficient corrosion resistance in the operation environment of the fuel cell. As the structure for improving corrosion resistance of the gas separator, the structure of coating the surface of the gas separator with silver has been proposed (e.g., Japanese Patent Laid-Open Publication No. SHO 60-115173). By coating the surface with silver, corrosion resistance of the metal gas separator can be significantly improved.
However, the internal environment of the operating fuel cell becomes highly acidic, thereby possibly making the corrosion resistance of the gas separator insufficient even in the case of the silver-coated metal gas separator. The internal environment of the fuel cell is considered to be acidified mainly by the following two factors: in the fuel cell (e.g., polymer electrolyte fuel cell), a catalyst layer including platinum, a platinum alloy or the like is provided on the surface of the electrolyte membrane. This catalyst layer normally contains a residual sulfate or the like of platinum that is used as a material for forming the catalyst layer. Accordingly, when the fuel cell is started, the residual platinum salt is eluted into the water produced in the gas flow path in the fuel cell, thereby acidifying the internal environment of the fuel cell. Moreover, the solid polymer electrolyte membrane provided in the polymer electrolyte fuel cell includes sulfonates as a functional group for realizing the proton conductivity. This solid polymer electrolyte membrane is gradually decomposed little by little at the portions of the sulfonates during power-generating operation of the fuel cell, thereby producing sulfuric acid. Thus, the internal environment of the fuel cell is acidified.
It is known that such platinum-salt elution and sulfonate decomposition as described above acidify the internal environment of the fuel cell to about pH 2. Under such strongly acidic conditions, the corrosion resistance of the gas separator may possibly become insufficient over the long-time operation of the fuel cell, even if the gas separator is coated with silver having a low ionization tendency. As the surface of the gas separator corrodes, the metal forming the gas separator is eluted as metal ions. Thus, if the metal ions (silver ions, or ions of a metal forming the substrate portion of the silver-coated separator) are eluted from the gas separator into the solid polymer electrolyte membrane even in a slight amount, such metal ions are attracted to the ion exchange groups (sulfonates) included in the electrolyte membrane, thereby degrading the proton conductivity of the solid polymer electrolyte membrane. This is not desirable for maintaining the performance of the fuel cell. Accordingly, a fuel cell gas separator with improved corrosion resistance has been desired.
The invention is made in view of the foregoing problems, and it is an object of the invention to provide a fuel cell gas separator for realizing sufficient corrosion resistance in a metal gas separator, a manufacturing method thereof, and a fuel cell.
In order to achieve the aforementioned object, a fuel cell gas separator according to one aspect of the invention includes a separator base material formed from a metal, a noble metal coating layer formed at least on a part of the separator base material, and a carbon coating layer formed on the noble metal coating layer. The noble metal coating layer is formed at least on the separator base material surface in a region of the gas separator that contacts an adjacent member of the fuel cell when the gas separator is integrated into the fuel cell, in other words, a region associated with a contact resistance corresponding to a contact surface that is in contact with the adjacent member.
A method for manufacturing a fuel cell gas separator according to another aspect of the invention includes the steps of (a) forming a separator base material having a predetermined shape from a metal, (b) forming a noble metal coating layer from a noble metal at least on a part of the separator base material formed in the step (b), i.e., at least on a region associated with a contact resistance with an adjacent member on a separator base material surface corresponding to a contact surface that is in contact with the adjacent member when the gas separator is integrated into the fuel cell, and (c) forming a carbon coating layer from a carbon material on the noble metal coating layer formed in the step (b).
A method for manufacturing a fuel cell gas separator according to still another aspect of the invention includes the steps of (a) forming a noble metal coating layer from a noble metal at least on a region of a surface of a metal member serving as a base material of the gas separator, (b) forming a carbon coating layer from a carbon material on the noble metal coating layer formed in the step (a), and (c) forming the metal member having both the noble metal coating layer and the carbon coating layer being formed on the surface thereof into a predetermined shape.
The fuel cell gas separator according to the aforementioned aspect of the invention as well as the fuel cell gas separators manufactured by the respective manufacturing methods according to the aforementioned aspects of the invention includes a noble metal coating layer formed from a noble metal. This noble metal coating layer is formed at least on a region associated with a contact resistance with an adjacent member on a separator base material surface corresponding to a contact surface that is in contact with the adjacent member when the gas separator is integrated into the fuel cell. Accordingly, in the metal forming such a separator, the region coated with the noble metal coating layer, i.e., the region associated with the conductivity of the fuel cell gas separator, is not oxidized to form a passive state film. As a result, increase in resistance resulting from the passive state film can be prevented.
Moreover, the noble metal forming the noble metal coating layer is a highly corrosion-resistant metal having a low ionization tendency. Therefore, in the fuel cell gas separator, sufficient corrosion resistance can be ensured in the region where such a noble metal coating layer is formed. In particular, the carbon coating layer of the carbon material is further formed on the noble coating layer of the noble metal. Therefore, extremely high corrosion resistance can be realized in the region where the noble metal coating layer is formed. Moreover, by providing the carbon coating layer on the noble metal coating layer, the noble metal coating layer is exposed to a milder environment (pH closer to neutral). Therefore, the required thickness of the noble metal coating layer for realizing sufficient corrosion resistance can be reduced. As a result, the manufacturing cost of the fuel gas separator can be reduced as compared to the case where the corrosion resistance is ensured only with the noble metal.
Note that the carbon coating layer formed from the carbon material need only contain the carbon material in such an amount that is capable of realizing sufficient conductivity. The carbon coating layer may further include a binder or the like for forming the layer as the carbon coating layer.
Moreover, in the fuel cell gas separator according to the aforementioned aspect of the invention and the manufacturing methods according to the aforementioned aspects of the invention, the structure of forming the carbon coating layer on the noble metal coating layer is not limited to the structure of forming the carbon coating layer directly onto the noble metal coating layer. It is also possible to interpose a coating layer between the noble metal coating layer and the carbon coating layer for the purpose of protecting the noble metal coating layer, improving adhesion between the noble metal coating layer and carbon coating layer, or the like. The invention is also applicable to such a structure.
In the fuel cell gas separator according to the aforementioned aspect of the invention, the noble metal coating layer may have a thickness in a range from 0.01 xcexcm to 10 xcexcm. Normally, the plating layer is not a uniform, smooth layer, and has small holes therein. Formation of such holes can be suppressed by increasing the plating thickness. However, in the metal plating, the effect of suppressing formation of the small holes normally reaches a saturated state when the thickness exceeds about 10 xcexcm. Therefore, by providing the noble metal layer having such a thickness, the metal forming the fuel cell gas separator and coated with the noble metal coating layer is prevented from being corroded through the small holes. As a result, corrosion resistance of the fuel cell gas separator can be effectively ensured. In particular, the fuel cell gas separator according to the aforementioned aspect of the invention includes the carbon coating layer formed from the carbon material. Therefore, the degree of corrosion resistance required for the noble metal coating layer is reduced, whereby sufficient corrosion resistance can be obtained even if the thickness of the noble metal coating layer is reduced to 1 xcexcm or less.
In the fuel cell gas separator of the invention as well as in the first and second manufacturing methods of the invention, the noble metal forming the noble metal coating layer may be silver. Silver is a relatively less noble metal in the noble metals. However, by providing the carbon coating layer thereon, silver itself can realize sufficient corrosion resistance. Moreover, silver is a relatively inexpensive metal in the noble metals. Therefore, the cost required for manufacturing the fuel cell gas separator having excellent corrosion resistance and conductivity can be reduced.
Also, the noble metal forming the noble metal coating layer may be gold. According to a separator having a noble metal coating layer formed from gold, even in a case that an internal environment of the fuel cell is made severer, such as a case that the fuel cell is operated at a higher temperature, the reliability regarding the corrosion resistance can be secured.
Moreover, the separator base material may be formed from a base metal. The carbon coating layer may be formed on a region forming the gas flow path within the fuel cell, in addition to the region where the noble metal coating layer is formed, on the separator base material. The base metal forming the separator base material may form a passive state film under a condition that the carbon coating layer is formed thereon.
With such a structure, the separator base material is formed from a base metal that may form a passive state film under the condition that the carbon coating layer is formed thereon. Therefore, sufficient corrosion resistance can be provided by also coating the region other than the region where the noble metal coating layer is formed. The base metal that forms a passive state film (which is an oxide film) is protected from corrosion by forming the passive state layer. Therefore, such a base metal has excellent corrosion resistance. Moreover, the corrosion resistance of such a base metal is further improved by forming the carbon coating layer of the carbon material thereon. Accordingly, by forming the carbon coating layer also on the region other than the region where the noble metal coating layer is formed, sufficient overall corrosion resistance of the fuel cell gas separator can be ensured. Note that an example of the base metal material having excellent corrosion resistance by forming the passive state film at its surface, and having sufficient strength as well as formability suitable for forming the separator base material is stainless steel.
The noble metal coating layer may further be formed on a region forming the gas flow path, in addition to the region associated with the contact resistance, on the separator base material. With such a structure, corrosion resistance can be ensured by both the noble metal coating layer formed from the noble metal and the carbon coating layer formed thereon, even in the region forming the gas flow path.
The fuel cell gas separator may further include an underlying coating layer formed from a base metal and formed between the noble metal coating layer and the separator base material at least on the region associated with the contact resistance in the separator base material.
Moreover, the base metal forming the underlying coating layer may be nobler, i.e., may have a lower ionization tendency, than the metal forming the separator base material. With such a structure, the noble metal coating layer of the noble metal can easily be formed even when the separator base material is formed from a base metal having a large ionization tendency. More specifically, since the base metal having a large ionization tendency may possibly be corroded by a noble metal plating bath, it is difficult to plate such a base metal with a noble metal. However, the noble metal plating can be facilitated by forming the underlying coating layer of a nobler base metal on the separator base material. In the case where different metal species are present, a less noble metal may be more likely to corrode. However, by providing the underlying coating layer of a nobler base metal, such an effect is suppressed, whereby the overall corrosion resistance of the separator can be ensured.
The carbon coating layer and the underlying coating layer may further be formed on a region forming the gas flow path within the fuel cell, in addition to the region associated with the contact resistance, on the separator base material. The underlying coating layer may be formed from a base metal that may form a passive state film under a condition that the carbon coating layer is formed thereon.
With such a structure, the underlying coating layer is formed from a base metal that may form a passive state film under a condition that the carbon coating layer is formed thereon. Therefore, by providing the underlying coating layer having the carbon coating layer thereon also on the region other than the region where the noble metal coating layer is formed, sufficient corrosion resistance can be obtained. The base metal that forms a passive state film (which is an oxide film) is protected from corrosion by forming the passive state layer. Therefore, such a base metal has excellent corrosion resistance. Moreover, the corrosion resistance is further improved by providing the carbon coating layer thereon. Accordingly, even if the noble metal coating layer is not provided in the region forming the gas flow path within the fuel cell, sufficient overall corrosion resistance of the fuel cell gas separator can be ensured by providing this region with the underlying coating layer having the carbon coating layer formed thereon.
The underlying coating layer may have a thickness in a range from 0.01 xcexcm to 10 xcexcm. Normally, the plating layer is not a uniform, smooth layer, and has small holes therein. Formation of such holes can be suppressed by increasing the plating thickness. However, in the metal plating, the effect of suppressing formation of the small holes normally reaches a saturated state when the thickness exceeds about 10 xcexcm. Therefore, by providing the underlying coating layer having such a thickness, the metal forming the fuel cell gas separator and coated with the underlying coating layer is prevented from being corroded through the small holes. As a result, corrosion resistance of the fuel cell gas separator can be effectively ensured.
In the manufacturing method according to the aforementioned aspect of the invention, the carbon coating layer may further include an acid-resistant resin or rubber as a binder, in addition to the carbon material. With such a structure, the effect of improving the corrosion resistance of the gas separator by providing the carbon coating layer on the fuel cell gas separator can further be enhanced. In other words, the binder as described above has excellent corrosion resistance, and also, can prevent the water from penetrating through the carbon coating layer formed from the carbon material. As a result, the metal forming the fuel cell gas separator can be prevented from being corroded by the water penetrating through the carbon coating layer.
A fuel cell gas separator according to a further aspect of the invention includes a separator base material formed from a metal, a base metal coating layer formed at least on a part of the separator base material, and a carbon coating layer formed on the base metal coating layer. The base metal coating layer is formed from a base metal, and formed at least on a region associated with a contact resistance with an adjacent member on the separator base material surface corresponding to a contact surface that is in contact with the adjacent member when the gas separator is integrated into the fuel cell. The base metal coating layer includes a plurality of electron-conductive particles at least at a surface that is in contact with the carbon coating layer. The electron-conductive particles are stable enough under a condition that the carbon coating layer is formed on the base metal coating layer.
A method for manufacturing a fuel cell gas separator according to a still further aspect of the invention includes the steps of (a) forming a separator base material having a predetermined shape from a metal, (b) forming a base metal coating layer from a base metal at least on a part of the separator base material formed in the step (a), i.e., at least on a region associated with a contact resistance with an adjacent member on a separator base material surface corresponding to a contact surface that is in contact with the adjacent member when the gas separator is integrated into the fuel cell, and (c) forming a carbon coating layer from a carbon material on the base metal coating layer formed in the step (b). The base metal coating layer formed in the step (b) includes a plurality of electron-conductive particles at least at a surface that is in contact with the carbon coating layer. The electron-conductive particles are stable enough under a condition that the carbon coating layer is formed on the base metal coating layer.
A method for manufacturing a fuel cell gas separator according to a yet further aspect of the invention includes the steps of (a) forming a base metal coating layer from a base metal at least on a region of a surface of a metal member serving as a base material of the fuel cell gas separator, (b) forming a carbon coating layer from a carbon material on the base metal coating layer formed in the step (a), and (c) forming the metal member having both the base metal coating layer and the carbon coating layer being formed on the surface thereof into a predetermined shape. The base metal coating layer formed in the step (a) includes a plurality of electron-conductive particles at least at a surface that is in contact with the carbon coating layer, the electron-conductive particles being stable enough under a condition that the carbon coating layer is formed on the base metal coating layer.
The fuel cell gas separator according to the aforementioned aspect of the invention as well as the gas separators manufactured by the respective manufacturing methods according to the aforementioned aspects of the invention include a base metal coating layer formed from a base metal. This base metal coating layer is formed at least on a region associated with a contact resistance with an adjacent member on a separator base material surface corresponding to a contact surface that is in contact with the adjacent member when the gas separator is integrated into the fuel cell. Moreover, this base metal coating layer includes a plurality of electron-conductive particles at least at a surface that is in contact with the carbon coating layer, and the electron-conductive particles are stable enough under a condition that the carbon coating layer is formed on the base metal coating layer. Accordingly, even if the conductivity is reduced in the base metal coating layer due to the fact that the metal forming the base metal coating layer is oxidized to form a passive state film, the conductivity of the separator is ensured by the electron-conductive particles. As a result, increase in resistance of the separator can be prevented.
The electron-conductive particles may be particles containing carbon.
Moreover, the base metal forming the base metal coating layer may be a metal whose surface may be oxidized to form a passive state layer under the condition that the carbon coating layer is formed on the base metal coating layer.
With such a structure, the base metal coating layer is formed from a base metal that forms a passive state film. Therefore, sufficient corrosion resistance can be provided in the region provided with the base metal coating layer. The base metal that forms a passive state film (which is an oxide film) is protected from corrosion by forming the passive state layer. Therefore, such a base metal has excellent corrosion resistance. Moreover, the corrosion resistance is further improved by forming the carbon coating layer on the base metal coating layer. Accordingly, in the region provided with the base metal coating layer, sufficient corrosion resistance in addition to the aforementioned conductivity can be realized.
The aspects of the invention are not limited to such gas separators and manufacturing method thereof as described above. For example, other aspects of the invention are formed as a fuel cell using the gas separator, a gas separator manufactured by the manufacturing method, and a fuel cell using the gas separator. A yet further aspect is formed as a vehicle provided with a fuel cell.